Process for after treatment of anodic oxide or chemical conversion coatings of aluminum or aluminum alloys

ABSTRACT

ANODIC OXIDE OR CHEMICAL CONVERSION COATINGS FORMED ON ALUMINUM OR ALUMINUM ALLOYS IN A CONVENTIONAL MANNER ARE SUBJECTED TO ELECTROLYSIS IN AN AQUEOUS SOLUTION CONTAINING AT LEAST ONE OF THE COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF CALCIUM AND MAGNESIUM SALT AS AN ELECTROLYTE WITH USE OF ELECTRIC CURRENT TO PRODUCE A CEMENT LIKE SUBSTANCE ON THE COATINGS THEREBY FORMING A SURFACE WHICH EXCELS IN RESISTANCE TO CORROSION AND STAINING.

United States Patent Ofice PROCESS FOR AFTER TREATMENT OF ANODIC OXIDE OR CHEMICAL CONVERSION COATINGS OF ALUMINUM OR ALUMINUM ALLOYS Ikno Mita, Hiroyuki Kawamura, and Shiro Manaka, Tokyo, Japan, assignors to Tokyo Kaken Kogyo Co, Ltd., Tokyo, Japan No Drawing. Filed Feb. 22, 1972, Ser. No. 228,295

Claims priority, application Japan, Sept. 8, 1971, 46/68,958 Int. Cl. C23b 11/02 US. Cl. 204-56 14 Claims ABSTRACT OF THE DISCLOSURE Anodic oxide or chemical conversion coatings formed on aluminum or aluminum alloys in a conventional manner are subjected to electrolysis in an aqueous solution containing at least one of the compounds selected from the group consisting of calcium and magnesium salts as an electrolyte with use of electric current to produce a cement like substance on the coatings thereby forming a surface which excels in resistance to corrosion and staining.

BACKGROUND OF INVENTION This invention relates to a process for after treating anodic oxide coatings or chemical conversion coatings of aluminum or aluminum alloys.

Aluminum and its alloys have come into extensive use in such applications as, architectural components, cooking utensils, decorative articles, optical parts, etc.

For such uses, all aluminum articles are usually subjected to surface treatments, such as chemical conversion or anodization to form oxide film on the surface of the article. However, the conventional anodic oxide coatings and chemical conversion coatings are defective in resistance to corrosion and staining. In order to improve the resistance of the anodic oxide coatings to corrosion and resistance to staining, various after treatments are performed, such as steam sealing with or without pressure, Water sealing or the like. However, sufficient improvement cannot be obtained using these conventional after treatments. Moreover, these conventional after treatments require heating or utilize pressure, and as a result, the conventional after treatments are difficult to adapt to continuous operation, particularly for handling large components, and are also expensive.

SUMMARY OF INVENTION Objects of this invention are to eliminate the above mentioned disadvantages of the prior art methods, and to provide a new process wherein surfaces excelling in resistances to corrosion and staining, as compared to those obtained by prior art methods, are obtainable simply, reproducibly and inexpensively.

The invention encompasses a new process for after treating oxide surfaces of aluminum or aluminum alloys, wherein the oxide surface is subjected to electrolysis in an aqueous solution containing at least one compound selected from the group consisting of calcium and magnesium salts, as an electrolyte.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Before proceeding with a detailed discussion of the invention, it may be helpful to define a few terms used herein.

The term oxide surface of aluminum or aluminum alloys, as used herein, refers to chemical conversion coat- 3,791,943. Patented Feb. 12., 1974 ings formed on the surface of aluminum and/or aluminum alloys, by conventional chemical conversions (e.g. the surface is dipped in a hot acid or alkaline solution) and/ or by anodic oxide coatings formed on the surface of aluminum and/or aluminum alloys by conventional anodizing. Thus, the term after treatment, as used herein, is to be understood to mean a treatment in which the above oxide surface is subjected to electrolysis in a special aqueous solution as electrolyte, and under conditions discussed above and more fully described hereinbelow.

According to the present invention, it is important that in order to obtain a surface of aluminum or its alloys having superior resistances to corrosion and staining, the oxide surface thereof be subjected to electrolysis in an electrolyte containing special alkaline metal salts, to thereby form a cementation surface on the oxide surface. This fact was arrived at by the inventors on the observation that at least one of the ions of calcium and/or magnesium present in the electrolyte, is bonded onto the oxide surface during the electrolysis, to produce a cement like substance on the oxide surface and to fill the pores of the oxide layer. Thus, the oxide surface is changed to a cementation surface having excellent resistances to corrosion and staining.

The special components or constituents of the electrolyte used in the present invention, are the calcium and/ or magnesium salts of mineral acid or organic carboxylic acid. For example, the salts may include calcium sulfates, magnesium sulfates, calcium carbonates, magnesium carbonates, basic magnesium carbonates, magnesium bicarbonates, calcium phosphates, magnesium phosphates, magnesium hydrogen phosphates, sodium calcium phosphates, magnesium diammonium hydrogen phosphates, magnesium hydroxides, calcium acetates, magnesium acetates, calcium oxalates, magnesium oxalates, calcium citrates, calcium benzoates and mixtures thereof.

Suitable mixtures of the constituents may include calcium carbonates-calcium phosphates, magnesium sulfatescalcium acetates, magnesium acetatescalcium carbonates, magnesium sulfates-calcium carbonates.

Concentration of the above constituents in the electrolyte depends upon tht solubility of the constituents in water, so that no particular value is assigned to the concentration. In general, the concentration may be such an amount that cement like substances are produced on the oxide surface by the electrolysis. Thus, when relativel; high solubility salts, or soluble salts, such as calcium sulfates, magnesium sulfates, etc., are used, it is preferable to have the range of solubility from about 0.2% to about 20%, and preferably about from 0.3% to 10%. When slightly soluble salts, such as calcium phosphates, calcium carbonates, are used, there are required to be used in a saturation state.

According to the present invention, the surface of aluminum may be pretreated by degreasing in a 5% canstic soda bath and neutralized in a nitric acid bath, and then subjected to either chemical conversion in a bath of 0.1 N sodium hydroxide, or to anodization in an electrolyte to produce an oxide surface of chemical conversion coatings or anodic oxide coatings, in a conventional manner. Thereafter, the oxide surface thus obtained is subjected to electrolysis in a saturated solution of or in a solution of about 0.2% to 20% aqueous solution of calcium and/or magnesium salts, wherein electric current is applied having a current density of about 0.1 to 2 a./dm. at about room temperature, for about 3 to 10 minutes, to absorb and bond Ca-ion and/or Mg-ion of the electrolyte onto the oxide surface, so as to deposit a cement like substance and consequently form a cementation surface onto the surface.

The electric current supplied to the electrol sis can be various kinds of current, such as alternating current, direct current, superimposed current having alternating current and direct current components and imperfectly rectified current.

The current density may be within the range of about 0.1 to 1 a./dm. preferably within the range of 0.2 to 0.6 a./dm. regardless of the direct current component and the alternating current component.

When current density is less than 0.1 a./dm. it was found ditficult to produce a cement like substance on the oxide surface, whereas, when current density is more than 1 a./dm. pitting is produced on the resulting surface. Thus, the current density should be within the range of about 0.1 to 1 a./dm.

The electrolytic reaction may be achieved at room temperature, at about atmospheric pressure, and for about 3 to 10 minutes. These parameters can be adjusted for optimum results.

Advantageously, surfaces resulting from the foregoing process have better resistances to corrosion and staining, than those of conventional chemical conversion coatings and anodic oxide coatings. Moreover, the present inventive process can be carried out easily and inexpensively, both being of highly valued commercial advantages. Furthermore, the calcium and magnesium salts, used as constituents in the present invention, are harmless to the human body and are inexpensive. Thus, these salts are valuable in the anodizing business. Furthermore, the inventive process has many other advantages, in that it can be made to operate continuously, requires short processing time, is performable at room temperature, and under atmospheric pressure.

This invention will now be illustratively set forth by the following actual examples. It is to be understood that the invention is not limited to such examples.

EXAMPLE 1 Pretreatment and anodizing Aluminum panels of size 50 x 100 x 1 mm. and grade A1050 PH24, were first degreased in a caustic soda bath, at about 70 C. for about seconds, rinsed, neutralized in a 10% nitric acid bath and then again rinsed sufliciently. The panels thus pretreated were subjected to anodizing in a sulfuric acid electrolyte of (by weight) H 50 and 5 g./l. of aluminum with a current density of about 1.0 a./dm. at about C., and for about 30 minutes, to obtain anodic oxide coatings. The formed coatings were rinsed and then the thickness of the anodic oxide coatings was measured. The thickness of each coating was about 8 microns.

For comparison, some of the anodized panels were sealed in hot water for about 60 minutes, as is done in conventional processes.

After treatment The panels thus anodized were subjected to electrolysis in a saturated solution of calcium carbonate, supplying a direct current having a current density of about 0.2 a./dm. at a temperature of about 30 C., for about 5 minutes, to produce a cement like substance on the surface of the anodic oxide coatings, so as to obtain cementation surfaces.

Resistances to corrosion and staining were tested on the panels having the cementation surface and compared with conventionally sealed ones.

Corrosion resistance Both the panels having the cementation surface obtained by using the inventive after treatment, and the panels having the conventionally sealed surfaces, were soaked in a 10% caustic soda solution at a temperature of 35 C.- -1 C., as specified by JISH8601, respectively. (118 refers to Japanese Industrial Standard.)

It was shown by tests that the cementation surfaces had no corrosion on them until 45 seconds had elapsed from the beginning, whereas the conventional surfaces started corroding within about 35 seconds. As can be understood, from the above description, the cementation surfaces obtained by using the inventive process, have longer noncorrosion periods than the conventionally obtained surfaces. Thus, the superiority of the cementation surfaces to corrosion resistance over the conventionally obtained surfaces was demonstrated markedly.

Staining resistance The cementation surfaces produced by the inventive after treatment and the surfaces produced by conventional after treatment, were soaked in a 10% solution of Aluminum First Red B3 LW, at a temperature of about 25 C., for about 5 minutes, so as to compare the resistances to staining. It was found that the cementation surfaces produced by the inventive process had no stain thereon, and proved to be perfectly sealed. On the other hand, the conventionally sealed surfaces were colored and were considerably worse than the surfaces of the present invention. Thus, it was demonstrated markedly that the inventive cementation surfaces were superior to the conventionally after treated surfaces.

EXAMPLE 2 Anodic oxide coatings obtained in the same manner as described in Example 1 were subjected to electrolysis in a saturated solution of calcium carbonate, supplying alternating current having current density of 0.5 a./dm. at a temperature of 30 C. and for 5 minutes, to produce a cement like substance on the coatings and consequently producing cementation surfaces.

The corrosion tests of the foregoing cementation surfaces were then carried out in the same manner as described in Example 1. The cementation surfaces were not corroded until about 52 seconds had elapsed from the beginning. Anodic oxide coatings processed by the conventional sealing in hot water and added ammonium, became corroded within about 40 seconds. It was thus shown that the corrosion resistance of the cementation surfaces of this invention was considerably better than that of the prior art anodic oxide coatings conventionally sealed.

Furthermore, the tests of resistances to staining were carried out on the cementation surfaces and the conventionally after treated surfaces, in the same manner as described in Example 1. The results showed that the cementation surfaces of this invention had no stain on it, whereas the conventionally treated surfaces were stained. Thus, it was markedly shown that the inventive surfaces had better resistance to staining than the conventionally after treated surfaces.

EXAMPLE 3 Anodic oxide coatings obtained in the same manner as in Example 1 were subjected to electrolysis in a saturated solution of calcium carbonate, supplying superimposed current comprising direct current component having current density of 0.2 a./dm. at a temperature of 20 C., for 5 minutes, to produce a cement like substance on the coatings and consequently produce cementation surfaces. Thereafter, tests of resistances to corrosion and stainmg were carried out on the cementation surfaces, in the same manner as described in Example 1. The results were substantially the same as those in Example 1.

EXAMPLE 4 Anodic oxide coatings obtained in the same manner as described in Example 1, were subjected to electrolysis n a saturated solution of calcium carbonate, supplying imperfectly rectified current comprising direct current component having current density of 0.1 a./dm. and an alternating current component having current density of 0.3 a./dm. at a temperature of 20 C., for 5 minutes, to produce a cement like substance on the coatings and consequently a cementation surface.

Tests of resistances to corrosion and staining were carried out on the cementation surface in the same manner as described in Example 1. The results were substantially the same as those in Example 1.

EXAMPLE 5 Anodic oxide coatings obtained in the same manner as described in Example 1, were subjected to electrolysis in 1% aqueous solution of magnesium sulfate, applying direct current having current density of 1 a./dm. at a temperature of 20 C., for 5 minutes, to produce a cement like substance on the coatings and consequently a cementation surface.

Tests of resistances to corrosion and staining were car ried out on the cementation surface in the same manner as described in Example 1. Substantially the same results as those in Example 1 were obtained.

EXAMPLE 6 Anodic oxide coatings obtained in the same manner as described in Example 1, were subjected to electrolysis in 0.5% aqueous solution of magnesium sulfate, suppling alternating current having current density of 0.6 a./dm. at a temperature of 20 C., for 5 minutes, to produce a cement like substance on the coatings and consequently a a cementation surface.

Tests of the resistances to corrosion and staining were carried out on the cementation surface in the same manner as described in Example 1. Substantially the same results as those in Example 1 were obtained.

EXAMPLE 7 Anodic oxide coatings obtained in the same manner as described in Example 1, were subjected to electrolysis in 0.5% aqueous solution of magnesium sulfate, supplying superimposed current comprising direct current component having current density of 1.0 a./dm. and alternating current component having current density of 0.1 a./ dmP, at a temperature of about 20 C., for about 5 minutes, to produce a cement like substance on the coating and consequently a cementation surface.

Tests for corrosion and staining resistances were performed in the same manner as in Example 1. The results were'substantially the same as in Example 1.

EXAMPLE 8 Anodic oxide coatings obtained in the same manner as described in Example 1, were subjected to electrolysis in 0.5 aqueous solution containing magnesium sulfate and calcium acetate, supplying direct current having a current density of 0.3 a./dm. at a temperature of 20 C., for 5 minutes, to produce a cement like substance on the coating and consequently a cementation surface.

Tests were performed thereon for resistances to corrosion and staining in the same manner as in Example 1. Substantially the same results were obtained as in Example 1.

EXAMPLE 9 Anodic oxide coatings obtained in the same manner as described in Example 1, were subjected to electrolysis in 0.3% aqueous solution of calcium acetate, supplying direct current having current density of 0.1 a./dm. at a temperature of 25 C., for 5 minutes, to produce a cement like substance on the coating and consequently a cementation surface.

Tests were performed thereon for resistances to corrosion and staining as in Example 1. The same results were obtained as in Example 1.

6 EXAMPLE 10 Chemical conversion treatment Aluminum panels of designation A6063S-T5, having the dimensions 70 x 150 x 1 mm. were subjected to degreasing by dipping in a 5.0% caustic soda bath at 70 C. for 10 seconds, rinsed, neutralized in a 30% nitric acid bath and again rinsed.

Thereafter, the panels thus treated were subjected to chemical conversion treatment in a sodium hydroxide bath at a temperature which was higher than C. for about 30 minutes to obtain chemical conversion coatings on the surface of the panels.

After treatment The chemical conversion coatings thus obtained were subjected to electrolysis in a saturated aqueous solution of calcium carbonate, supplying alternating current having current density of 0.2 a./dm. at a temperature of 23 C., for 5 minutes, to produce a cement like substance on the coatings and consequently a cementation surface.

A copper-accelerated acetic acid salt spray test (CASS test) was carried out on the cementation surface thus obtained, according to the specification of JIS-H-860l. The R.N. of the cementation surface obtained according to this invention, was about 8 to 9. The R.N. of the above chemical conversion coatings which were not-after-treated according to the present inventive process was about 7. This demonstrated markedly that the performance in this test of the cementation surface treated according to this invention was considerably better than that of the conventional chemical conversion coatings not subjected to the invention process.

EXAMPLE 11 Chemical conversion coatings obtained in the same manner as described in Example 10 were subjected to electrolysis in a 0.5% aqueous solution of magnesium sulfate, supplying alternating current having current density of 0.6 a./dm. at a temperature of 23 C., for 5 minutes, to produce a cement like substance on the coatings and consequently cementation surfaces.

The cementation surfaces thus obtained were tested in accordance with the CASS test, in the same manner as described in Example 10. The R.N. of the cementation surfaces of the invention was about 9. The R.N. of the above chemical conversion coatings which were not after treated in accordance with the inventive process was about 7, the same as in Example 10.

The foregoing disclosure is intended to be only illustrative of the invention. Numerous variations and modifications thereof would be apparent to one skilled in the art. All such modifications and variations are to be considered to be within the spirit and scope of the invention.

What is claimed is:

1. A process for after treating chemical conversion or anodic oxide coating of aluminum and/or aluminum alloys, comprising the steps of subjecting said coating to electrolysis in an aqueous solution consisting essentially of a concentration of 0.2 to 20% of at least one compound selected from the group consisting of calcium and magnesium salts as an electrolyte, supplying an electric current of a current density of 0.1 to 1 a./dm. thereto, to produce a cement-like substance on said coating, said electric current being AC, DC, superimposed AC and DC or imperfectly rectified current.

2. Process according to claim 1, wherein said compound is selected from the group consisting of calcium sulfates, magnesium sulfates, calcium carbonates, magnesium carbonates, calcium phosphates, magnesium phosphates, calcium acetates, magnesium acetates, calcium oxalates, magnesium oxalates and mixtures thereof.

3. Process of claim 2, wherein said mixtures include calcium carbonates-calcium phosphates, magnesium sulfates-calcium acetates, magnesium acetates-calcium carbonates and magnesium sulfates-calcium carbonates.

4. Process of claim 1, wherein said salts are slightly soluble an used in a saturated aqueous solution.

5. Process of claim 1, wherein said salts are relatively highly souble in said aqueous solution.

6. Process of claim 5, wherein said concentration is from 0.3% to 10%.

7. Process according to claim 1, wherein said current density is from 0.2 to 0.6 a./dm.

8. Process of claim 1, wherein said compound is selected from the group consisting of basic magnesium carbonates, magnesium bicarbonates, magnesium hydrogen phosphates, sodium calcium phosphates, magnesium diammonium hydrogen phosphates, magnesium hydroxides, calcium citrates, calcium benzoates and mixtures thereof.

9. Process of claim 1, wherein said electrolysis is performed at about room temperature to about 30 C.

10. Process of claim 1, wherein said electrolysis is performed for about 3 to 10 minutes.

11. Process of claim 10, wherein said time period is about 5 minutes.

12. Process for after treating an anodic oxide or chemical conversion coating of aluminum or alloys thereof, comprising the steps of subjecting said coating to electrolysis in a saturated aqueous solution consisting essentially of at least one compound selected from the group consisting of salts of calcium and magnesium, said electrolysis using an electric current having a current density of from 0.1 to 1 a./dm. at substantially room temperature, and for a time ranging from about 3 minutes to about 10 minutes, said electric current being AC, DC, superimposed AC and DC or imperfectly rectified DC.

13. Process of claim 12, wherein said current density is from 0.2 to 0.6 a./dm.

14. Process of claim 12, wherein said concentration is from 0.3 to 10%.

References Cited UNITED STATES PATENTS 2,578,400 12/1951 Cohn 20458 3,616,309 10/1971 Asada 204-58 OTHER REFERENCES The Surface Treatment and Finishing of Al, Wernick et al., 1964, p. 181.

JOHN H, MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-38 B, 40, 58 

